llvmpipe: lower hadd/add_sat

Fixes some CL piglits.

Acked-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -599,6 +599,8 @@ static const struct nir_shader_compiler_options gallivm_nir_options = {
    .lower_sub = true,
    .lower_ffma = true,
    .lower_fmod = true,
+   .lower_hadd = true,
+   .lower_add_sat = true,
    .lower_pack_snorm_2x16 = true,
    .lower_pack_snorm_4x8 = true,
    .lower_pack_unorm_2x16 = true,